The present invention relates to the use of titanium dioxide mixtures in the anatase form which have defined physical properties for producing catalysts which are suitable, in particular, for the synthesis of phthalic anhydride. The invention further relates to catalysts comprising titanium dioxide mixtures in the anatase form which have defined physical properties.
Catalysts for preparing phthalic anhydride which comprise vanadium pentoxide and titanium dioxide have been known for a long time. Titanium dioxide in the anatase modification is the main constituent of the active composition of these phthalic anhydride catalysts and serves as support for the catalytically active and selective vanadium pentoxide components.
DE-A 2 106 796 describes the production of supported catalysts for the oxidation of o-xylene to phthalic anhydride, in which the titanium dioxide has a BET surface area of from 15 to 100 m2/g, preferably from 25 to 50 m2/g. A mixture of anatase having a BET surface area of from 7 to 11 m2/g and hydrated titanium dioxide having a BET surface area of >100 m2/g is particularly suitable, while the components alone would not be suitable.
EP-A 522 871 describes a relationship between the BET surface area of the titanium dioxide and the catalyst activity. According to this document, the catalyst activity is low when titanium dioxide having BET surface areas of less than 10 m2/g is used. When titanium dioxide having a BET surface area of greater than 60 m2/g is used, the life of the catalyst is reduced and the phthalic anhydride yield decreases sharply. Preference is given to BET surface areas of from 15 to 40 m2/g.
To improve the phthalic anhydride yield and the starting behavior of the catalysts, a change has been made in recent years to the use of activity-structured catalysts. The individual catalyst zones are structured so that the activity of the individual zones generally increases from the reactor inlet to the reactor outlet.
For example, EP-A 985 648 describes the preparation of phthalic anhydride by catalytic gas-phase oxidation of o-xylene and/or naphthalene using a catalyst system which is structured so that the porosity of the catalyst and thus the activity increases pseudocontinuously from the reactor inlet to the reactor outlet. The porosity is defined as the free volume between the coated shaped bodies of the bed in the reaction tube. In the examples, the specific surface area of the active components was altered by varying the specific surface area of the titanium dioxide, which was in the range from 40 to 140 m2/g.
According to the prior art summarized in EP-A1 063 222, the activity can be increased in very different ways:    (1) by means of a continual increase in the phosphorus content,    (2) by means of a continual increase in the content of active composition,    (3) by means of a continual decrease in the alkali metal content,    (4) by means of a continual decrease in the empty space between the individual catalysts,    (5) by means of a continual decrease in the content of the inert materials or    (6) by means of a continual increase in the temperature
from the upper zone (reactor inlet) to the bottom zone (reactor outlet). The BET surface area of the titanium dioxide should be in the range from 10 to 60 m2/g. In the examples of EP-A1 063 222, the BET surface area is constant at 22 m2/g.
In multizone catalyst systems, the decrease in the activity of the first catalyst zone has an adverse effect on the life of the catalyst. With increasing aging, the conversion in the region of the first highly selective zone decreases. Over the course of the catalyst life, the main reaction zone migrates ever deeper into the catalyst bed, i.e. the o-xylene or naphthalene feed is to an increasing extent reacted only in the subsequent less selective zones. The consequences are reduced phthalic anhydride yields and an increased concentration of by-products or unreacted starting materials. To avoid migration of the main reaction zone into the subsequent zones, the salt bath temperature can be continually increased. However, as the period of operation of the catalysts increases, this measure, too, leads to a reduction in the phthalic anhydride yield.